Selective plasma etching of silicon oxide relative to silicon nitride by gas pulsing

ABSTRACT

A method for selective plasma etching of silicon oxide relative to silicon nitride. The method includes a) providing a substrate containing a silicon oxide film and a silicon nitride film, b) exposing the substrate to a plasma-excited treatment gas containing 1) H2 and 2) HF, F2, or both HF and F2, to form a silicon oxide surface layer with reduced oxygen content on the silicon oxide film and form an ammonium salt layer on the silicon nitride film, c) exposing the substrate to a plasma-excited halogen-containing gas that reacts with and removes the silicon oxide surface layer from the silicon oxide film, and d) repeating steps b) and c) at least once to further selectively etch the silicon oxide film relative to the ammonium salt layer on the silicon nitride film. The ammonium salt layer may be removed when the desired etching has been achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims priority to U.S. ProvisionalPatent Application Ser. No. 62/793,893 filed on Jan. 18, 2019, theentire contents of which are herein incorporated by reference. Thisapplication is related to and claims priority to U.S. Provisional PatentApplication Ser. No. 62/926,754 filed on Oct. 28, 2019, the entirecontents of which are herein incorporated by reference.

FIELD OF INVENTION

The present invention relates to the field of semiconductormanufacturing and semiconductor devices, and more particularly, to amethod of selective plasma etching of silicon oxide relative to siliconnitride in semiconductor manufacturing.

BACKGROUND OF THE INVENTION

Next generation semiconductor technology development poses a hugechallenge as dry etch removal of one material selective to othermaterials is needed, for example in the etching of <5 nm contacts. Insilicon oxide or silicon nitride etching by plasmas containingfluorocarbon (FC) or hydrofluorocarbon (HFC) gases, a fluorinated carbonlayer is typically formed on the film surface. The thickness, density,and atomic compositions of such a carbon layer depend on the kineticenergies and fluxes of incident ions and charge-neutral species from theplasma and strongly affect the etch rate of the material beneath it.

SUMMARY OF THE INVENTION

A method of selective plasma etching of silicon oxide relative tosilicon nitride in semiconductor manufacturing is disclosed in severalembodiments.

According to one embodiment, the method includes a) providing asubstrate containing a silicon oxide film and a silicon nitride film, b)exposing the substrate to a plasma-excited treatment gas containing 1)H₂ and 2) HF, F₂, or both HF and F₂, to form a silicon oxide surfacelayer with reduced oxygen content on the silicon oxide film and form anammonium salt layer on the silicon nitride film, c) exposing thesubstrate to a plasma-excited halogen-containing gas that reacts withand removes the silicon oxide surface layer from the silicon oxide film,and d) repeating steps b) and c) at least once to further selectivelyetch the silicon oxide film relative to the ammonium salt layer on thesilicon nitride film. According to one embodiment, the method furtherincludes removing the ammonium salt layer from the silicon nitride filmwhen the desired etching of the silicon oxide film has been achieved.

According to another embodiment, the method includes a) providing asubstrate containing a silicon oxide film and a silicon nitride film, b)exposing the substrate to a plasma-excited H₂-containing gas to form asilicon oxide surface layer with reduced oxygen content on the siliconoxide film and form a hydrogenated silicon nitride surface layer on thesilicon nitride film, c) exposing the substrate to a plasma-excitedhalogen-containing gas that reacts with and removes the silicon oxidesurface layer from the silicon oxide film and forms an ammonium saltlayer from the hydrogenated silicon nitride surface layer, and d)repeating steps b) and c) at least once to further selectively etch thesilicon oxide film relative to the ammonium salt layer on the siliconnitride film. According to one embodiment, the method further includesremoving the ammonium salt layer from the silicon nitride film when thedesired etching of the silicon oxide film has been achieved.

DETAILED DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A-1G schematically show through cross-sectional views a method ofselective plasma etching of a silicon oxide film relative to a siliconnitride film by gas pulsing according to an embodiment of the invention;and

FIGS. 2A-2G schematically show through cross-sectional views a method ofselective plasma etching of a silicon oxide film relative to a siliconnitride film by gas pulsing according to another embodiment of theinvention.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

A method of selective plasma etching of silicon oxide relative tosilicon nitride by gas pulsing is described. Embodiments of theinvention utilize diverging surface chemical reactions of silicon oxideand silicon nitride films by sequential plasma exposures of A) 1) H₂ and2) HF, F₂, or both HF and F₂, and B) a halogen-containing gas atsubstrate temperature and gas pressure conditions that achieve selectiveetching of the silicon oxide film relative to the silicon nitride film.The method includes repeated cycles of a two-step process that isdescribed with respect to surface reactions on the silicon oxide andsilicon nitride films, leading to selective etching of the silicon oxidefilm. According to one embodiment, in the first step A), a plasmaexposure of 1) H₂ and 2) HF, F₂, or both HF and F₂, is performed thatresults in chemical surface reduction of the silicon oxide film andformation of an ammonium salt layer on the silicon nitride film.Thereafter, in the second step B), a halogen-containing plasma treatmentis performed that results in formation and desorption of volatilebyproducts from the reduced silicon oxide film. At the end of each cycleof the two-step process, a portion of the silicon oxide film is removed,a silicon oxide film surface is restored, while the layer of an ammoniumsalt that is formed on silicon nitride film serves as a blocking layer.

According to one embodiment, in the first step, a H₂ plasma treatment isperformed that results in chemical surface reduction of the siliconoxide film and chemical surface hydrogenation of the silicon nitridefilm. Thereafter, in the second step, a halogen-containing plasmatreatment is performed that results in formation and desorption ofvolatile byproducts from the reduced silicon oxide film and halogenincorporation into the surface hydrogenated silicon nitride film to forman ammonium salt layer. At the end of each cycle of the two-stepprocess, a portion of the silicon oxide film is removed, a silicon oxidefilm surface is restored, while the layer of an ammonium salt that isformed on silicon nitride film that serves as an etch blocking layer.

The inventive selective plasma etching process of silicon oxide relativeto silicon nitride described in embodiments of the inventionfundamentally differs from conventional silicon oxide or silicon nitrideetching process by plasmas containing fluorocarbon (FC) orhydrofluorocarbon (HFC) gases. This allows for selective passivation onthe Si₃N₄ film relative to the SiO₂ film. In the inventive process, anammonium salt layer latter is formed in a bottom-up manner with nitrogen(N) provided from the hydrogenated Si₃N₄ surface layer or the Si₃N₄film, whereas in the conventional etching process, a fluorinated carbonlayer is formed in a top-down manner on the substrate surface during theetching process.

The method of selective plasma etching of SiO₂ films relative to Si₃N₄films may be performed in conventional commercial plasma processingsystems, including Inductively Coupled Plasma (ICP) systems,Capacitively Coupled Plasma (CCP) systems, remote plasma systems thatgenerate plasma excited species upstream from the substrate, electroncyclotron resonance (ECR) systems, and other systems.

FIGS. 1A-1G schematically show through cross-sectional views a method ofselective plasma etching of a SiO₂ film relative to a Si₃N₄ film by gaspulsing according to an embodiment of the invention. The method combinesselective ammonium salt formation on the Si₃N₄ film and selectivechemical reduction of the SiO₂ film to provide highly controllable andselective SiO₂/Si₃N₄ etching.

FIG. 1A schematically shows a substrate 1 containing a SiO₂ film 100 anda Si₃N₄ film 120. In the example shown in FIG. 1A, the SiO₂ film 100 andthe Si₃N₄ film 120 are in the same horizontal plane, but embodiments ofthe invention may also be applied to films that are not in the samehorizontal plane but are offset vertically. Si₃N₄ is the mostthermodynamically stable of the silicon nitrides and hence the mostcommercially important of the silicon nitrides. However, embodiments ofthe invention may be applied to other silicon nitrides that contain Siand N as the major constituents, where the silicon nitrides can have awide range of Si and N compositions. Similarly, SiO₂ is the mostthermodynamically stable of the silicon oxides and hence the mostcommercially important of the silicon oxides. However, embodiments ofthe invention may be applied to other silicon oxides that contain Si andO as the major constituents, where the silicon oxides can have a widerange of Si and O compositions.

The method includes exposing the substrate 1 to a plasma-excitedtreatment gas 101 that is hydrogen-fluorine-based as is schematicallyshown in FIG. 1B. The treatment gas can contain or consist of 1) H₂ andHF; 2) H₂, HF, and a noble gas, 3) H₂ and F₂, 4) H₂, F₂, and a noblegas, 5) H₂, HF, and F₂, or 6) H₂, HF, F₂, and a noble gas (e.g., argon(Ar) or helium (He)). Hydrogen ions (H⁺) and hydrogen radicals (H.) inthe plasma-excited treatment gas 101 react with the SiO₂ film 100 andfacilitate oxygen (O) removal via O—H bond formation and subsequentformation and desorption of volatile H₂O from the SiO₂ film 100. Theplasma exposure forms a SiO_(x) (x<2) surface layer 102 on the SiO₂ film100, where the SiO_(x) (x<2) surface layer 102 has low O-content andhigh Si-content relative to the SiO₂ film 100. The hydrogen ions (H⁻),hydrogen radicals (H.), and plasma excited HF, F₂, or both HF and F₂,react with the Si₃N₄ film 120 to form an ammonium salt layer 122containing NH₄F and (NH₄)₂SiF₆. It is contemplated that the exposure tothe plasma excited treatment gas forms Lewis bases such as —NH— and —NH₂surface species, as well as NH₃ molecules. These Lewis basessubsequently react with acidic HF species to form the ammonium saltlayer 122. FIG. 1C schematically shows the substrate 1 following theexposure to the plasma-excited treatment gas 101.

Thereafter, the method further includes exposing the substrate 1 to ahalogen-containing plasma. In one embodiment, the halogen-containingplasma can include a plasma-excited halogen-containing gas 103, wherethe plasma-excited halogen-containing gas 103 reacts with and removesthe SiO_(x) surface layer 102 from the SiO₂ film 100 but the ammoniumsalt layer 122 protects the underlying Si₃N₄ film 120 from etching.Possible volatile reaction products include SiX₄ and CO_(a)X_(b). Thisis schematically shown in FIGS. 1D and 1E. According to one embodiment,the halogen-containing gas can include a fluorine-containing gas (e.g.CF₄, CHF₃, CH₂F₂, CH₃F, NF₃, or SF₆, or a combination thereof), achlorine-containing gas (e.g. Cl₂, CCl₄, SiCl₄, SiH₂Cl₂, or CHCl₃, or acombination thereof), a bromine-containing gas (e.g. HBr, Br₂, or SiBr₄,or a combination thereof), or a combination thereof. Thehalogen-containing gas can further include an additive gas (e.g. O₂, N₂,or CO₂, or a combination thereof). In some examples, thehalogen-containing gas can include HBr and O₂, Cl₂ and O₂, or HBr, Cl₂and O₂. According to another embodiment, the halogen-containing gas caninclude a fluoro-carbon-based gas that includes a fluorocarbon gas, ahydrofluorocarbon gas, or a combination thereof. The fluorocarbon-basedgas may further contain a non-polymerizing gas (e.g., Ar, He, SF₆, or acombination thereof). For example, the fluorocarbon gas can contain orconsist of CF₄, and the hydrofluorocarbon gas can contain or consist ofCHF₃, CH₂F₂, CH₃F, or a combination thereof. The sequential andalternating steps of exposing the substrate 1 to the plasma-excitedtreatment gas 101 and exposing the substrate 1 to the plasma-excitedhalogen-containing gas 103 may be repeated at least once to furtherselectively etch the SiO₂ film 100 relative to the ammonium salt layer122 on the Si₃N₄ film 120. The resulting substrate 1 is shown in FIG.1F. The steps of pulsing the plasma-excited treatment gas 101 and theplasma-excited halogen-containing gas 103 may be interrupted by gaspurging steps that remove the plasma species and any gaseous reactionbyproducts from the plasma chamber.

According to one embodiment, the method further includes removing theammonium salt layer 122 from the Si₃N₄ film 120 (e.g., by wet cleaning,heating, or IR irradiation) when the desired etching of the SiO₂ film100 has been achieved. The resulting substrate 1 is shown in FIG. 1G.According to one embodiment, the method further includes, following theremoval of the ammonium salt layer 122 from the Si₃N₄ film 120,repeating the sequential and alternating steps of exposing the substrate1 to the plasma-excited treatment gas 101 and exposing the substrate 1to the plasma-excited halogen-containing gas 103 at least once tofurther selectively etch the SiO₂ film 100, and thereafter, removing theammonium salt layer 122 from the Si₃N₄ film 120.

Halogen species generated in the plasma-excited halogen-containing gas103 react with the SiO surface layer 102 to form volatile SiX₄ species(where X is a halogen), and CO_(a)X_(b) species when using acarbon-containing gas (e.g., CCl₄ or CHCl₃), that desorb from thesubstrate 1 and thereby etch the SiO surface layer 102 and restore aSiO₂ surface. In contrast, the ammonium salt layer 122 protects theunderlying Si₃N₄ film 120 from etching by the plasma-excitedhalogen-containing gas 103. Thus, the ammonium salt layer 122 functionsas an etch stop layer (blocking layer) that hinders or prevents furthermodification/etching of the underlying Si₃N₄ film 120 when the steps ofexposing the substrate 1 to the plasma-excited treatment gas 101, andexposing the substrate 1 to a plasma-excited halogen-containing gas 103,are repeated at least once to further selectively etch the SiO₂ film 100relative to the Si₃N₄ film 120.

The selective SiO₂/Si₃N₄ etching process may be performed at substratetemperatures and gas pressures that optimize 0 removal from the SiO₂film 100 by the plasma-excited treatment gas 101, optimize the ammoniumsalt formation on the Si₃N₄ film 120, and optimize the etching of theSiO surface layer 102 by the plasma-excited halogen-containing gas 103.Examples include a substrate temperature between about −100° C. andabout 25° C., between about −100° C. and about −30° C., between about−100° C. and about 0° C., between about −30° C. and about 25° C., orbetween about 0° C. and about 25° C. The gas pressure in the plasma etchchamber can between about 10 mTorr and about 500 mTorr, between about 10mTorr and 200 mTorr, or between about 20 mTorr and about 100 mTorr.Further, the use of low energy ions in the plasma that impinge on thesubstrate 1 provides good SiO₂/Si₃N₄ etch selectivity, provides goodcontrol over the SiO₂ etch depth per cycle, avoids or reduces excessivephysical sputtering of Si₃N₄ film 120 and the ammonium salt layer 124,and reduces the ion implantation depth. Still further, the addition ofthe non-polymerizing gas may be used during the exposure to thecarbon-containing gas to enhance the removal of the SiO_(x) surfacelayer 102.

FIGS. 2A-2G schematically show through cross-sectional views a method ofselective plasma etching of a SiO₂ film relative to a Si₃N₄ film by gaspulsing according to another embodiment of the invention. The methodcombines selective ammonium salt formation on the Si₃N₄ film andselective chemical reduction of the SiO₂ film to provide highlycontrollable and selective SiO₂/Si₃N₄ etching.

FIG. 2A schematically shows a substrate 2 containing a SiO₂ film 200 anda Si₃N₄ film 220. In the example shown in FIG. 2A, the SiO₂ film 200 andthe Si₃N₄ film 220 are in the same horizontal plane, but embodiments ofthe invention may also be applied to films that are not in the samehorizontal plane but are offset vertically.

The method includes treating the substrate 2 with a H₂-based plasma thatcan include plasma-excited H₂-containing gas 201 as is schematicallyshown in FIG. 2B. The H₂-containing gas can, for example, consist of H₂,or H₂ and a noble gas (e.g., argon (Ar) or helium (He)).

Hydrogen ions (H⁺) and hydrogen radicals (H.) in the plasma-excitedH₂-containing gas 201 react with the SiO₂ film 200 and facilitate oxygen(O) removal via O—H bond formation and subsequent formation anddesorption of volatile H₂O from the SiO₂ film 200. The plasma exposureforms a SiO_(x) (x<2) surface layer 202 on the SiO₂ film 200, where theSiO_(x) (x<2) surface layer 202 has low O-content and high Si-contentrelative to the SiO₂ film 200. In contrast, the hydrogen ions (H⁺) andthe hydrogen radicals (H.) react with the Si₃N₄ film 220 by surfacehydrogenation/protonation rather than nitrogen (N) removal, therebyforming a hydrogenated Si₃N₄ surface layer 222 that is ammonium (NH₄⁺)-rich. FIG. 2C schematically shows the substrate 1 following theexposure to the plasma-excited H₂-containing gas 201.

Thereafter, the method further includes exposing the substrate 2 to aplasma-excited halogen-containing gas 203, where the exposing reactswith and removes the SiO_(x) surface layer 202 from the SiO₂ film 200and forms an ammonium salt layer 224 by reacting with the hydrogenatedsilicon nitride surface layer 222. This is schematically shown in FIGS.2D and 2E. Some of the different halogen-containing gases and additivegases that may be used were described above in reference to FIG. 1D.

The sequential and alternating steps of treating the substrate 2 withthe plasma-excited H₂-containing gas 201 and exposing the substrate 2 tothe plasma-excited halogen-containing gas 203 may be repeated at leastonce to further selectively etch the SiO₂ film 200 relative to theammonium salt layer 224 on the Si₃N₄ film 220. The resulting substrate 2is shown in FIG. 2F. The steps of pulsing the plasma-excitedH₂-containing gas 201 and the plasma-excited halogen-containing gas 203may be interrupted by gas purging steps that remove the plasma speciesand any gaseous reaction byproducts from the plasma chamber. Accordingto one embodiment, the method further includes removing the ammoniumsalt layer 224 from the Si₃N₄ film 220 when the desired etching of theSiO₂ film 100 has been achieved (e.g. by wet cleaning, heating, or IRirradiation). The resulting substrate 2 is shown in FIG. 2G. Accordingto one embodiment, the method further includes, following the removingof the ammonium salt layer 224 from the Si₃N₄ film 220, repeating thesequential and alternating steps of exposing the substrate 2 with theplasma-excited H₂-containing gas 201 and exposing the substrate 2 to theplasma-excited halogen-containing gas 203 at least once to furtherselectively etch the SiO₂ film 200, and thereafter, removing theammonium salt layer 224 from the Si₃N₄ film 120.

Halogen species generated in the plasma-excited halogen-containing gas203 react with the SiO, surface layer 202 to form volatile SiX₄ (where Xis a halogen), and CO_(a)X_(b) species when using a carbon-containinggas, that desorb from the substrate 2 and thereby etch the SiO_(x)surface layer 202 and restore a SiO₂ surface. In contrast, the speciesgenerated in the plasma-excited halogen-containing gas 203 react withthe hydrogenated Si₃N₄ surface layer 222 to form the ammonium salt layer224 that functions as an etch stop layer (blocking layer) that hindersor prevents further modification/etching of the underlying Si₃N₄ film220 when the steps of treating the substrate 2 with the plasma-excitedH₂-containing gas 201, and exposing the substrate 2 to theplasma-excited halogen-containing gas 203, are repeated at least once tofurther selectively etch the SiO₂ film 200 relative to the Si₃N₄ film220.

The selective SiO₂/Si₃N₄ etching process may be performed at substratetemperatures and gas pressures that optimize O removal from the SiO₂film 200 by the plasma-excited H₂-containing gas 201, and optimize theammonium salt formation on the Si₃N₄ film 220 by the plasma-excitedhalogen-containing gas 203. Examples include a substrate temperaturebetween about −100° C. and about 25° C., between about −100° C. andabout −30° C., between about −100° C. and about 0° C., between about−30° C. and about 25° C., or between about 0° C. and about 25° C. Thegas pressure in the plasma etch chamber can between about 10 mTorr andabout 500 mTorr, between about 10 mTorr and 200 mTorr, or between about20 mTorr and about 100 mTorr. Further, the use of low energy of ions inthe plasma that impinge on the substrate 2 provides good SiO₂/Si₃N₄ etchselectivity, provides good control over the SiO₂ etch depth per cycle,avoids or reduces excessive physical sputtering of Si₃N₄ film 220 andthe ammonium salt layer 224, and reduces the ion implantation depth.Still further, the addition of a non-polymerizing gas may be used duringthe exposure to the carbon-containing gas to enhance the removal of theSiO_(x) surface layer 202.

A plurality of embodiments for a method of selective plasma etching ofsilicon oxide relative to silicon nitride in semiconductor manufacturinghave been described. The foregoing description of the embodiments of theinvention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. This description and theclaims following include terms that are used for descriptive purposesonly and are not to be construed as limiting. Persons skilled in therelevant art can appreciate that many modifications and variations arepossible in light of the above teaching. Persons skilled in the art willrecognize various equivalent combinations and substitutions for variouscomponents shown in the Figures. It is therefore intended that the scopeof the invention be limited not by this detailed description, but ratherby the claims appended hereto.

1. A plasma processing method, comprising: a) providing a substratecontaining a silicon oxide film and a silicon nitride film; b) exposingthe substrate to a plasma-excited treatment gas containing 1) H₂ and 2)HF, F₂, or both HF and F₂, to form a silicon oxide surface layer withreduced oxygen content on the silicon oxide film and form an ammoniumsalt layer on the silicon nitride film; c) exposing the substrate to aplasma-excited halogen-containing gas that reacts with and removes thesilicon oxide surface layer from the silicon oxide film; and d)repeating steps b) and c) at least once to further selectively etch thesilicon oxide film relative to the ammonium salt layer on the siliconnitride film.
 2. The method of claim 1, wherein the plasma-excitedtreatment gas contains or consists of 1) H₂ and HF; 2) H₂, HF, and anoble gas; 3) H₂ and F₂; 4) H₂, F₂, and a noble gas; 5) H₂, HF, and F₂;or 6) H₂, HF, F₂, and a noble gas.
 3. The method of claim 1, wherein thesilicon oxide film includes SiO₂ and the silicon oxide surface layerincludes SiO_(x), where x<2.
 4. The method of claim 1, wherein thesilicon nitride film includes Si₃N₄.
 5. The method of claim 1, whereinthe ammonium salt layer includes NH₄F, (NH₄)₂SiF₆, or a combinationthereof.
 6. The method of claim 1, wherein the plasma-excitedhalogen-containing gas includes CF₄, CHF₃, CH₂F₂, CH₃F, NF₃, SF₆, Cl₂,CCl₄, SiCl₄, SiH₂Cl₂, CHCl₃, HBr, Br₂, or SiBr₄, or a combinationthereof.
 7. The method of claim 1, wherein the plasma-excitedhalogen-containing gas includes a fluorocarbon gas, a hydrofluorocarbongas, or a combination thereof.
 8. The method of claim 1, wherein theplasma-excited halogen-containing gas contains CF₄, CHF₃, CH₂F₂, orCH₃F, or a combination thereof.
 9. The method of claim 1, furthercomprising; following step d), e) removing the ammonium salt layer fromthe silicon nitride film.
 10. The method of claim 9, further comprisingf) repeating steps b)-e) at least once to further selectively etch thesilicon oxide film.
 11. A plasma processing method, comprising: a)providing a substrate containing a silicon oxide film and a siliconnitride film; b) exposing the substrate to a plasma-excitedH₂-containing gas to form a silicon oxide surface layer with reducedoxygen content on the silicon oxide film and form a hydrogenated siliconnitride surface layer on the silicon nitride film; c) exposing thesubstrate to a plasma-excited halogen-containing gas that reacts withand removes the silicon oxide surface layer from the silicon oxide filmand forms an ammonium salt layer from the hydrogenated silicon nitridesurface layer; and d) repeating steps b) and c) at least once to furtherselectively etch the silicon oxide film relative to the ammonium saltlayer on the silicon nitride film.
 12. The method of claim 11, whereinthe plasma-excited H₂-containing gas consists of H₂, or H₂ and a noblegas.
 13. The method of claim 11, wherein the silicon oxide film includesSiO₂ and the silicon oxide surface layer includes SiO_(x), where x<2.14. The method of claim 11, wherein the silicon nitride film includesSi₃N₄.
 15. The method of claim 11, wherein the ammonium salt layerincludes NH₄X, (NH₄)₂SiX₆, or a combination thereof, where X is ahalogen.
 16. The method of claim 11, wherein the plasma-excitedhalogen-containing gas includes CF₄, CHF₃, CH₂F₂, CH₃F, NF₃, SF₆, Cl₂,CCl₄, SiCl₄, SiH₂Cl₂, CHCl₃, HBr, Br₂, or SiBr₄, or a combinationthereof.
 17. The method of claim 11, wherein the plasma-excitedhalogen-containing gas includes a fluorocarbon gas, a hydrofluorocarbongas, or a combination thereof.
 18. The method of claim 11, wherein theplasma-excited halogen-containing gas contains CF₄, CHF₃, CH₂F₂, CH₃F,or a combination thereof.
 19. The method of claim 11, furthercomprising; following step d), e) removing the ammonium salt layer fromthe silicon nitride film.
 20. The method of claim 19, further comprisingf) repeating steps b)-e) at least once to further selectively etch thesilicon oxide film.